He cant sell any more because he's already so high with his price, but he should be able to lower it down a bit if he wanted to I agree. There are two ways for you to solve the problem1. sell the huge amount of food needed or2. buy all cheaper food on marketThose are the only options I think...

well, to piggy back this topic, I find it ridiculous that you can't post resources at their max value when there are none on the market....for instance, I can't sell slaves for 300 a piece because of that 25% rule...I can only sell for 125 a piece. There are no slaves on the market, by default it should let me sell at any price between 0 and 300(the max allowed price).

Solution for Orin : Buy the cheaper food, probably only need to buy the lowest lot to fix your problem.

Solution for Fantamos : Sell one slave at 125, then one slave at 155, then one slave at 175, then one slave at 189, then one at 200. Problem solved. (Or if you wanna go to three hundred, 1 at 211, 20*219, and then 50*261. then rest at 300)